return(n) Tcl Built-In Commands return(n)
NAME
return - Return from a procedure, or set return code of a script
SYNOPSIS
return ?result?
return ?-code code? ?result?
return ?option value ...? ?result?
DESCRIPTION
In its simplest usage, the return command is used without options in
the body of a procedure to immediately return control to the caller of
the procedure. If a result argument is provided, its value becomes the
result of the procedure passed back to the caller. If result is not
specified then an empty string will be returned to the caller as the
result of the procedure.
The return command serves a similar function within script files that
are evaluated by the source command. When source evaluates the con-
tents of a file as a script, an invocation of the return command will
cause script evaluation to immediately cease, and the value result (or
an empty string) will be returned as the result of the source command.
EXCEPTIONAL RETURN CODES
In addition to the result of a procedure, the return code of a proce-
dure may also be set by return through use of the -code option. In the
usual case where the -code option is not specified the procedure will
return normally. However, the -code option may be used to generate an
exceptional return from the procedure. Code may have any of the fol-
lowing values:
ok (or 0) Normal return: same as if the option is omitted. The
return code of the procedure is 0 (TCL_OK).
error (or 1) Error return: the return code of the procedure is 1
(TCL_ERROR). The procedure command behaves in its calling
context as if it were the command error result. See below
for additional options.
return (or 2)
The return code of the procedure is 2 (TCL_RETURN). The
procedure command behaves in its calling context as if it
were the command return (with no arguments).
break (or 3) The return code of the procedure is 3 (TCL_BREAK). The
procedure command behaves in its calling context as if it
were the command break.
continue (or 4)
The return code of the procedure is 4 (TCL_CONTINUE). The
procedure command behaves in its calling context as if it
were the command continue.
value Value must be an integer; it will be returned as the
return code for the current procedure.
When a procedure wants to signal that it has received invalid arguments
from its caller, it may use return -code error with result set to a
suitable error message. Otherwise usage of the return -code option is
mostly limited to procedures that implement a new control structure.
The return -code command acts similarly within script files that are
evaluated by the source command. During the evaluation of the contents
of a file as a script by source, an invocation of the return -code code
command will cause the return code of source to be code.
RETURN OPTIONS
In addition to a result and a return code, evaluation of a command in
Tcl also produces a dictionary of return options. In general usage,
all option value pairs given as arguments to return become entries in
the return options dictionary, and any values at all are acceptable
except as noted below. The catch command may be used to capture all of
this information -- the return code, the result, and the return options
dictionary -- that arise from evaluation of a script.
As documented above, the -code entry in the return options dictionary
receives special treatment by Tcl. There are other return options also
recognized and treated specially by Tcl. They are:
-errorcode list
The -errorcode option receives special treatment only when the
value of the -code option is TCL_ERROR. Then the list value is
meant to be additional information about the error, presented as
a Tcl list for further processing by programs. If no -errorcode
option is provided to return when the -code error option is pro-
vided, Tcl will set the value of the -errorcode entry in the
return options dictionary to the default value of NONE. The
-errorcode return option will also be stored in the global vari-
able errorCode.
-errorinfo info
The -errorinfo option receives special treatment only when the
value of the -code option is TCL_ERROR. Then info is the ini-
tial stack trace, meant to provide to a human reader additional
information about the context in which the error occurred. The
stack trace will also be stored in the global variable error-
Info. If no -errorinfo option is provided to return when the
-code error option is provided, Tcl will provide its own initial
stack trace value in the entry for -errorinfo. Tcl's initial
stack trace will include only the call to the procedure, and
stack unwinding will append information about higher stack lev-
els, but there will be no information about the context of the
error within the procedure. Typically the info value is sup-
plied from the value of -errorinfo in a return options dictio-
nary captured by the catch command (or from the copy of that
information stored in the global variable errorInfo).
-errorstack list
The -errorstack option receives special treatment only when the
value of the -code option is TCL_ERROR. Then list is the ini-
tial error stack, recording actual argument values passed to
each proc level. The error stack will also be reachable through
info errorstack. If no -errorstack option is provided to return
when the -code error option is provided, Tcl will provide its
own initial error stack in the entry for -errorstack. Tcl's
initial error stack will include only the call to the procedure,
and stack unwinding will append information about higher stack
levels, but there will be no information about the context of
the error within the procedure. Typically the list value is
supplied from the value of -errorstack in a return options dic-
tionary captured by the catch command (or from the copy of that
information from info errorstack).
-level level
The -level and -code options work together to set the return
code to be returned by one of the commands currently being eval-
uated. The level value must be a non-negative integer repre-
senting a number of levels on the call stack. It defines the
number of levels up the stack at which the return code of a com-
mand currently being evaluated should be code. If no -level
option is provided, the default value of level is 1, so that
return sets the return code that the current procedure returns
to its caller, 1 level up the call stack. The mechanism by
which these options work is described in more detail below.
-options options
The value options must be a valid dictionary. The entries of
that dictionary are treated as additional option value pairs for
the return command.
RETURN CODE HANDLING MECHANISMS
Return codes are used in Tcl to control program flow. A Tcl script is
a sequence of Tcl commands. So long as each command evaluation returns
a return code of TCL_OK, evaluation will continue to the next command
in the script. Any exceptional return code (non-TCL_OK) returned by a
command evaluation causes the flow on to the next command to be inter-
rupted. Script evaluation ceases, and the exceptional return code from
the command becomes the return code of the full script evaluation.
This is the mechanism by which errors during script evaluation cause an
interruption and unwinding of the call stack. It is also the mechanism
by which commands like break, continue, and return cause script evalua-
tion to terminate without evaluating all commands in sequence.
Some of Tcl's built-in commands evaluate scripts as part of their func-
tioning. These commands can make use of exceptional return codes to
enable special features. For example, the built-in Tcl commands that
provide loops -- such as while, for, and foreach -- evaluate a script
that is the body of the loop. If evaluation of the loop body returns
the return code of TCL_BREAK or TCL_CONTINUE, the loop command can
react in such a way as to give the break and continue commands their
documented interpretation in loops.
Procedure invocation also involves evaluation of a script, the body of
the procedure. Procedure invocation provides special treatment when
evaluation of the procedure body returns the return code TCL_RETURN.
In that circumstance, the -level entry in the return options dictionary
is decremented. If after decrementing, the value of the -level entry
is 0, then the value of the -code entry becomes the return code of the
procedure. If after decrementing, the value of the -level entry is
greater than zero, then the return code of the procedure is TCL_RETURN.
If the procedure invocation occurred during the evaluation of the body
of another procedure, the process will repeat itself up the call stack,
decrementing the value of the -level entry at each level, so that the
code will be the return code of the current command level levels up the
call stack. The source command performs the same handling of the
TCL_RETURN return code, which explains the similarity of return invoca-
tion during a source to return invocation within a procedure.
The return code of the return command itself triggers this special han-
dling by procedure invocation. If return is provided the option -level
0, then the return code of the return command itself will be the value
code of the -code option (or TCL_OK by default). Any other value for
the -level option (including the default value of 1) will cause the
return code of the return command itself to be TCL_RETURN, triggering a
return from the enclosing procedure.
EXAMPLES
First, a simple example of using return to return from a procedure,
interrupting the procedure body.
proc printOneLine {} {
puts "line 1" ;# This line will be printed.
return
puts "line 2" ;# This line will not be printed. }
Next, an example of using return to set the value returned by the pro-
cedure.
proc returnX {} {return X} puts [returnX] ;# prints "X"
Next, a more complete example, using return -code error to report
invalid arguments.
proc factorial {n} {
if {![string is integer $n] || ($n < 0)} {
return -code error \
"expected non-negative integer,\
but got \"$n\""
}
if {$n < 2} {
return 1
}
set m [expr {$n - 1}]
set code [catch {factorial $m} factor]
if {$code != 0} {
return -code $code $factor
}
set product [expr {$n * $factor}]
if {$product < 0} {
return -code error \
"overflow computing factorial of $n"
}
return $product }
Next, a procedure replacement for break.
proc myBreak {} {
return -code break }
With the -level 0 option, return itself can serve as a replacement for
break, with the help of interp alias.
interp alias {} Break {} return -level 0 -code break
An example of using catch and return -options to re-raise a caught
error:
proc doSomething {} {
set resource [allocate]
catch {
# Long script of operations
# that might raise an error
} result options
deallocate $resource
return -options $options $result }
Finally an example of advanced use of the return options to create a
procedure replacement for return itself:
proc myReturn {args} {
set result ""
if {[llength $args] % 2} {
set result [lindex $args end]
set args [lrange $args 0 end-1]
}
set options [dict merge {-level 1} $args]
dict incr options -level
return -options $options $result }
SEE ALSO
break(n), catch(n), continue(n), dict(n), error(n), errorCode(n),
errorInfo(n), proc(n), source(n), throw(n), try(n)
KEYWORDS
break, catch, continue, error, exception, procedure, result, return
Tcl 8.5 return(n)
tcl 8.6.1 - Generated Tue Oct 1 07:41:33 CDT 2013